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<H1>Technical info</H1>

<CITE>
"Known?" said Gandalf.<BR>
"I have known much that only the Wise know, Frodo."
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Frodo tries to exactly imitate C64 hardware features. Now the 64's hardware
(esp. the graphics chip "VIC-II") has a rather simple design resulting in
many of the internal processes coming to the "outside". So there are lots
of "undocumented features" you can do effects with the designers never
dared to dream about.<P>

Frodo uses a line-by-line emulation, i.e. the function of the VIC and the
processor (6510) are emulated for one raster line of the C64 screen at
times. In practice, Frodo runs VIC and 6510 alternately for 63 simulated
cycles each (corresponding to one raster line). At first, it emulates the
processor for 63 cycles, then switches over to the VIC that paints one
pixel row to the screen, then again 63 cycles processor, and so on... If
the 1541 processor emulation is turned on, 6510 and 6502 (in the 1541)
instructions are executed by Frodo in an interleaved fashion.<P>

Even though this is a heavy simplification of the processes in a real C64,
it lets you perfectly emulate many graphical effects possible on the C64,
e.g. FLD, DYCP, hyperscreen and many more. But this method has one big
disadvantage: Changes made to VIC registers by the processor in the middle
of a raster line will only take effect at the start of the next line. E.g.
you can't change the border color in the middle of a line, the color change
takes place in the next line. Therefore, very sophisticated techniques
depending on the exact position of a register change can't be emulated. For
instance, it is no problem to open the top and bottom border, but opening
the left and right border is impossible (and therefore not implemented in
the emulation).<P>

Frodo SC goes one step further by switching between VIC and 6510 in every
cycle and precisely emulating the internal functions. Modifications to
VIC registers become visible immediately in the next clock phase and
therefore it can even emulate effects that depend on the exact position
of a register change within a raster line, e.g. special FLI routines,
opening the left/right border, linecrunch, DMA delay, multiple repeated
sprite lines and executing programs in open address spaces ($de00-$dfff)
and in the color RAM. The 6510 emulation is also more precise and does
the same memory accesses as the real 6510, even the "unnecessary" ones
that come from design weaknesses of the 6510 and are not needed for the
function of single opcodes (e.g. in an instruction sequence like
INX:INX:INX:INX, the 6510 reads every opcode twice).<P>

A detailed technical description of the VIC-II can be found in an
<A HREF="http://www.uni-mainz.de/~bauec002/VIC-Article.gz">article</A>
I wrote (32k gzipped).

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